Air pollution control system

ABSTRACT

An air pollution control system for treating the flue gases from a fossil fuel fired furnace scrubs the gases with a solution of NaOH whereby Na and HSO3 are formed in solution. The HSO3 is then oxidized to form H and SO4 ions. The solution is then causticized with CaO to form NaOH and precipitate CaSO4. The precipitated CaSO4 and the fly ash collected in the scrubber are then separated from the NaOH solution. The solution is then diluted with make-up water to form the NaOH scrubbing solution for recycle to the scrubber.

United States Patent [191 Klein et a1.

11] 3,870,781 1 Mar. 11, 1975 AIR POLLUTION CONTROL SYSTEM [73]Assignee: Combustion Engineering, Inc.,

Windsor, Conn.

[22] Filed: July 27, 1972 [21] App]. N0.: 275,662

[52] US. Cl. 423/242, 423/166 [51] Int. Cl C0lb 17/00 [58] Field ofSearch 423/242-244,

[56] References Cited UNITED STATES PATENTS 1,271,899 7/1918 Howard eta1. 423/242 2,161,056 6/1939 Johnstone et a1 423/242 PrimaryExaminer0scar R. Vertiz Assistant Examiner-Gregory A. Heller Attorney,Agent, or Firm-Richard H. Berneike [57] ABSTRACT An air pollutioncontrol system for treating the flue gases from a fossil fuel firedfurnace scrubs the gases with a solution of NaOH whereby Na and H80 areformed in solution. The H80 is then oxidized to form H and SO, ions. Thesolution is then causticized with CaO to form NaOH and precipitate CaSOThe precipitated CaSO, and the fly ash collected in the scrubber arethen separated from the NaOH solution. The solution is then diluted withmake-up water to form the NaOH scrubbing solution for recycle to thescrubber.

6 Claims, 3 Drawing Figures BACKGROUND OF THE INVENTION The discharge ofpollutants into the atmosphere has become of increasing concern as therate of discharge increases and as the level of pollutants in theatmosphere increases. One of the sources of air pollution is the sulfuroxides and particulate matter which are discharged with the flue gasesduring the combustion of sulfur-containing fuels. A number of systemshave been developed or proposed to remove the sulfur oxides andparticulate matter from the flue gases. One of the first commercialsystems involves scrubbing the flue gases with an aqueous solution whichcontains materials which will react with the sulfur oxides. The mostcommonly used materials are limestone and dolomite which may be addedeither to the furnace or directly to the scrubbing solution. However,unless specific measures are taken and very careful controls maintainedon the system chemistry, precipitates form in the scrubber and thesystem piping which tend to plug up the system and may eventually forceshut down.

SUMMARY oF THE INVENTION The present invention relates to an airpollution control system and more specific to a system in which thegases containing sulfur oxides are scrubbed with an aqueous solutioncontaining an alkali metal hydroxide whereby the sulfur oxides arereacted with the hydroxides to form bisulfites. The invention furtherinvolves the conversion of the bisulfites to sulfate and hydrogen ionsand the reaction of the sulfate and hydrogen ions with alkaline earthmetal compounds to precipitate alkaline earth metal sulfates and reformthe alkali metal hydroxide scrubbing solution. Other features of theinvention and the advantages will be apparent from the description whichfollows.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a flow diagram illustratingthe present invention and FIGS. 2 and 3 are flow diagrams whichillustrate modifications of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG.1, the flue gases containing sulfur oxides and fly ash enter thescrubber through the duct 12 and the cleaned gases exit through duct 14.The scrubber may be of the marble bed type or any other suitablescrubber for liquid-gas contact and fly ash removal. The scrubbingsolution, which contains NaOH and CaSO enters the scrubber from line 16and is contacted with the flue gas. The scrubbing solution recirculationrate is set or adjusted such that sufficient NaOH will always be presentto remove all of the sulfur oxides by the following reaction:

This is accomplished by providing an excess of NaOH above that requiredto react with the S0 in the flue gas. If only the stoichiometric amountof NaOH were provided, the resulting pH of the scrubber solution wouldbe about 4.5. Sufficient excess NaOH is provided such that the resultingpH will be in the range of about 5.0 to 5.6. If larger quantities ofNaOH were used and the pH were higher, there would be a tendency to formsulfites instead of bisulfites which could result in precipitation andscaling. Also, in the pH range of 5.0 to 5.6, there will be little COabsorbed from the flue gas. I

The spent scrubbing solution containing NaHSO CaSO, and residual NaOH insolution as well as fly ash leaves the scrubber through line I8 and isintroduced into the oxidizer 20. In the oxidizer the spent scrubbingsolution is contacted with air from line 22 in a countercurrent fashionwhereby the bisulfite in the solution is oxidized to sulfuric acid andNa SO by the following reaction:

The formation of hydrogen ions causes the pH to drop to about 2.0 andany residual CO is driven off and exists the oxidizer through line 24along with the excess air. Make-up water and make-up sodium sulfate toreplace the water and sodium lost with the discharged solids are alsoadded to the oxidizer 20 through line 26. The solution which is nowacidic leaves the oxidizer through line 28 where it enters thecausticizer 30.

Limestone is introduced into the kiln 32 from line 33. The CaCO isconverted to CaO in the kiln which is then introduced into thecausticizer 30 via line 34 where it is converted to the hydroxide. Theflue gas from the kiln may be discharged to the economizer of the boilerby line 35 for heat recovery. The following reaction takes place in thecausticizer:

Na in so: 2OH= caso.1+ Na OH H2O It can be seen that this reactioncauses the precipitation of the CaSO and regenerates an NaOH solution.

The slurry leaves the causticizer 30 through line 36 and enters theclarifier 38 in which the CaSO, and fly ash solids are concentrated. Thesupernatant solution is removed from the clarifier 38 through line 40and is sent to the hold tank 42. The concentrated solids from theclarifier 38 are sent through line 43 to the centrifuge 44 or someotherfiltering device. Some make-up water is introduced into the centrifuge44 through line 46 as wash water to recover NaOH from the solids. Thesolids from the centrifuge are discharged through line 48. These solidswhich contain primarily CaSO fly ash and water together with a smallamount of residual NaOH solution may be disposed of in any desiredmanner. The wash water from the centrifuge 44, which now contains someNaOH, is sent to the hold tank 42 through line 50.

The remainder of the make-up water which is necessary to compensate forthe water discharged with the saturated flue gas is added to the holdtank 42 through line 52 so as to regenerate the dilute scrubbingsolution. The scrubbing solution is then returned to the scrubber 10through line 16 to complete the cycle. NaOH which is lost from the cyclewith the solids is also added as required to the hold tank 42 throughline 54.

FIG. 2 illustrates a modification of the present invention in which theoxidizing and caustisizing steps are combined in one unit 56. Followingis a table which illustrates a typical material balance for the presentinvention as applied to the FIG. 2 embodiment. The numbers at the headof each column correspond to the numbers in FIG. 2 and the figures ineach column represent the weight in pounds per minute of each componentflowing in the various lines.

l2 l4 l6 18 26 33 34 36 40 42 48 5O 52 Flue gas 28,508 28,500 SO 12 Asii200 200 200 200 200 H20 900 57,038 56,lg) 472 56,5;2 55,l g 1,412 47294g 900 N OH 12155) 272 272 272 c s o. 160 160 162 156 r 6 2 4 Soln.NaHSO; 195 CaCO 200 CaO 112 Na SO 2 FIG. 3 illustrates a furtherembodiment of the inven 15 react with Said 2 y excess N H l tion inwhich a slaker 58 is included so that there will w ys be present andwherein the pH 18 maintained be a more complete utilization of the CaOfrom the kiln 1n the range of 5.0 to 5.6;

32. Also, a series of causticizers 30A, 30B and 30C are b. contactingsaid aqueous scrubbing solution from employed so as to assure thecomplete conversion to step (a) with oxygen whereby said bisulfite ionsare NaOH. 20 oxidized to form hydrogen ions and sulfate ions insolution;

The present invention eliminates all sulfites from the contacting Saidaqueous Scrubbing sohmon f system since these would be a source ofchemical oxy- Step (b) with CaO whereby C3504 is precipitated gen demandand thus objectionable from a water polluand NaOH is formed in Solution;tion standpoint. Also, the elimination of soluble sulfites Separatingprecipitated (321504 from Said aqueous prevents any possibility of SOback pressure in the sys- Scrubbing solution; and tem which would reducethe effect of the system in rerecycling Said aqueous Scrubbing solutionto Snap ducing SO from the flue gas. A further result of the (a).elimination of the sulfite ions is that scaling and plug- 2 A method asrecited in claim 1 and further includ ging problems are avoided. The airoxidation of the ing the Step of adding makemp Water and Nazso4 toscrubbei effluent ehmlriates CO2 from the y i thus said aqueousscrubbing solution subsequent to step (a) preventing loss of alkali dueto CaCO precipitation. and prior to p (C) while prefrred embodlments ofthe pre.sem i i 3. A method as recited in claim 1 and further includtionand specific examples have been described, it will g the p of washingSaid Separated precipitated be Linderstood that Flilanges may bemade.wltho.ut de' CaSO with water and returning said washing water toparting from the spirit and scope of the invention as step Clalmed' 4. Amethod as recited in claim 3 and further includ- What Clalmed ing thestep of adding additional make-up water to said 1. A method of removingS0 from a flue gas stream recycled aqueous Scrubbing Solution comprlsmgt F of: 40 s. A method as recited in claim 1 wherein the pH foli a.contacting said flue gas with an aqueous scrubbing lowing Step (b) isabout 9 conFammg NaOH @1594 whereby a 6. A method as recited in claim 1and further includ- 9 Q reactF sfild S02 to ing the step of adding NaOHmake-up to said recycled sodium ions and bisulfite ions, said NaOH beingaqueous Scrubbing Solution prior to step (a). present in an amountgreater than that required to e

1. A METHOD OF REMOVING SO2 FROM A FLUE GAS STREAM COMPRISING THE STEPSOF: A. CONTACTING SAID FLUE GAS WITH AS AQUEOUS SCRUBBING SOLUTIONCONTAINING NAOH AND CASO4 WHEREBY A PORTION OF SAID NAOH REACTS WITHSAID SO2 TO FORM SODIUM IONS AND BISULFITE IONS, SAID NAOH BEING PRESENTIN AN AMOUNT GREATER THAN THAT REQUIRED TO REC ACT WITH SAID SO2 WHEREBYEXCESS NAOH WILL ALWAYS BE PRESENT AND WHEREIN THE PH IS MAINTAINED INTHE RANGE OF 5.0 TO 5.6; B. CONTACTING SAID AQUEOUS SCRUBBING SOLUTIONFROM STEP (A) WITH OXYGEN WHEREBY SAID BISULFITE IONS ARE OXIDIZED TOFORM HYDROGEN IONS AND AND SULFATE IONS IN SOLUTION; C. CONTACTING SAIDAQUEOUS SCRUBBING SOLUTION FROM STEP (B) WITH CAO WHEREBY CASO4 ISPRECIPITATED AND NAOH IS FORMED IN SOLUTION; D. SEPARATING PRECIPITATEDCASO4 FROM SAID AQUEOUS SCRUBBEING SOLUTION; AND E. RECYCLING SAIDAQUEOUS SCRUBBING SOLUTION TO STEP (A).
 1. A method of removing SO2 froma flue gas stream comprising the steps of: a. contacting said flue gaswith an aqueous scrubbing solution containing NaOH and CaSO4 whereby aportion of said NaOH reacts with said SO2 to form sodium ions andbisulfite ions, said NaOH being present in an amount greater than thatrequired to react with said SO2 whereby excess NaOH will always bepresent and wherein the pH is maintained in the range of 5.0 to 5.6; b.contacting said aqueous scrubbing solution from step (a) with oxygenwhereby said bisulfite ions are oxidized to form hydrogen ions andsulfate ions in solution; c. contacting said aqueous scrubbing solutionfrom step (b) with CaO whereby CaSO4 is precipitated and NaOH is formedin solution; d. separating precipitated CaSO4 from said aqueousscrubbing solution; and e. recycling said aqueous scrubbing solution tostep (a).
 2. A method as recited in claim 1 and further including thestep of adding make-up water and Na2SO4 to said aqueous scrubbingsolution subsequent to step (a) and prior to step (c).
 3. A method asrecited in claim 1 and further including the step of washing saidseparated precipitated CaSO4 with water and returning said washing waterto step (a).
 4. A method as recited in claim 3 and further including thestep of adding additional make-up water to said recycled aqueousscrubbing solution.
 5. A method as recited in claim 1 wherein the pHfollowing step (b) is about 2.0.